Gurney flaps were originally used on racecars by Dan Gurney and have also been proposed for aircraft wings. They are mounted at the trailing-edge, normal to the chord and are typically around 1% of the relevant chord length (Liebeck, R. H., “Design of Subsonic Airfoils for High Lift,” AIAA Journal of Aircraft, Vol. 15, No. 9, 1978, pp. 547-561.). They increase maximum lift and in some instances, if they are short enough (<1% of chord), they can slightly increase aerodynamic efficiency.
Fans and propellers are widely used in many industrial applications. Fans are used for personal, industrial and automotive cooling, ventilation, in air-conditioning units, vacuuming and dust removal, inflating, etc. Propellers are most common on personal and commercial aircraft, while propellers and ducted, or shrouded, fans are also the propulsors for airboats, air-cushion vehicles, airships and model aircraft. Shrouded propellers and fans are also touted as the primarily propulsive system for so-called personal air vehicles, a number of which are under intense development. In recent years, propellers and ducted fans have received renewed attention, particularly for the propulsion of small-scale (typically ˜500 mm) unmanned air vehicles.
There is at present a strong trend toward the design of even smaller air vehicles, known as “Micro Air Vehicles” (MAVs) having maximum dimension typically between 7.5 cm and 15 cm), for a variety of military and civil applications. One consequence of these smaller scales and relatively low tip speeds is a reduced propeller fan blade Reynolds number, typically less than 100,000. At these Reynolds numbers, boundary layer transition does not occur and the boundary layer is susceptible to separation, which can result in a catastrophic loss of propulsion. The best performing blade profiles are thin, curved sections which do not produce large dynamic performance across the disk since leading-edge separation occurs at relatively low inflow angles.
Reynolds number is defined as: (air density)*(air speed relative to blade)*(chord length)/air dynamic viscosity. Thus, for a fan, as the distance from the hub increases, the Reynolds number increases proportionately because the “air speed relative to blades” increases with linearly radial distance from the hub.
Another application of fans and ducted fans at these length and velocity scales, is their ubiquitous use for the cooling of modern high-speed computer chips, motherboards and power supplies. The efficacy of these fans is often quantified as the ratio of Cubic Feet per Minute (CFM) to power input in Watts. Typically, a fan blows air across a heat-sink that is attached to a particular component, such as a CPU. In modern designs, fan speed can be controlled based on temperature feedback, and this is generally referred to as active cooling. However, modern high-speed processors require continuously greater cooling and this is generally accomplished using larger heat sinks and more powerful fans running at higher rounds per minute (rpm).
Apart from physical size limitations, these fans are increasingly noisy and require greater input power. In fact, the noise generated by fans that are used to cool high-end processors, particularly within a small physical computer sizes, is often objectionable to the user. In mobile and laptop computer, fan power requirement add to battery drain and reduces work-time between batteries charging.
The vast majority of commercial and industrial fans (computer cooling fans, personal upright and ceiling fans, refrigeration fans, air conditioning fans, automotive fans, ventilation, vacuuming etc.) are of relatively small dimension (from a few centimeters to about one meter) and they rotate at relatively low speeds (from several hundred to several thousand rpm). The blade chord lengths are also relatively small, typically from several millimeters to several centimeters). This all conspires to produce very low Reynolds numbers on the fan blades, from about 1000 to 100,000, where conventional aerodynamic shapes perform very poorly. (Small-scale propellers, such as those used for unmanned air vehicles, also suffer from similar problems.) The only airfoils that perform well at these Reynolds numbers are flat plates and curved plates and this has remained so for many decades with only minor variations on the theme.
Therefore, there is clearly a need for, and it would be highly advantageous to have, a fan; ducted fan or propeller having enhanced performance. Preferably, the performance enhancement would be achieved with a minimum or no increase of energy input.
U.S. Pat. No. 5,088,665; to Vijgen, Paul, et. al.; titled “Serrated trailing edges for improving lift and drag characteristics of lifting surfaces”; discloses a serrated trailing edge to enhance lift and drag of wing surface which has saw-tooth serrations with 60 deg. included angle between adjacent teeth.
U.S. Pat. No. 5,294,080; to Ross, James; titled “Lift enhancing tabs for airfoils”; discloses a device for enhancing lift of aircraft wing which includes tab deployable from trailing edge of fixed main element with members for actuating the tab to move from non-deployed position to deployed position.
U.S. Pat. No. 5,492,448; to Perry Frederick and Brocklehurst Alan; titled “Rotary blades” discloses rotary blades for generating lift and/or propulsion which comprises fixed boundary layer control device extending spanwise on lower surface adjacent trailing edge.
U.S. Pat. No. 6,015,115; to Dorsett, Kenneth Merlean and Stewart, Christopher Sean; titled “Inflatable structures to control aircraft” discloses an inflatable structure to control aircraft and a method of modifying the shape of an aircraft airfoil before and during flight, which includes inflating or deflating at least one inflatable bladder positioned on an aircraft wing. The inflatable structures to control aircraft include an aircraft wing structure, at least one inflatable section positioned on an aircraft wing structure in at least one location selected from the group of an upper surface, a lower surface, a leading edge and a trailing edge of the aircraft wing structure.
U.S. Pat. No. 6,863,245; to Gessler, Andreas, et. al.; titled “Aerodynamic profile with an adjustable flap”; discloses an arodynamic profile, with a trailing swing landing flap, has the flap at the under side of the lower skin and flush with it, pivoting on an integrated and airtight joint.
U.S. Pat. No. 7,028,954; to Van Dam, Cornelis P, et. al.; titled “Microfabricated translational stages for control of aerodynamic loading”; discloses a micro-fabricated translational stages for control of aerodynamic load on aircraft wing using micro-tabs mounted near or at trailing edge of wing to control lift.
U.S. Pat. No. 7,059,833; to Bonus Energy; titled “Method for improvement of the efficiency of a wind turbine rotor”; discloses a wind turbine rotor blade with a flexible serrated trailing edge.